1. Field of the Invention
The present invention relates to a method and apparatus for detecting a surface of a liquid in a container, and to a method and apparatus for use in an automated blood/plasma sampling system for detecting a surface of a liquid in a container and for controlling a position of a probe with respect to the surface of the liquid. More particularly, the present invention relates to a method and apparatus for maintaining accuracy of the liquid detection over a period of time.
An example of an automated blood/plasma sampling system for which the present invention is applicable is disclosed, for example, in U.S. Pat. No. 5,236,666 to Hulette et al. The subject matter of this patent to Hulette et al. is incorporated herein by reference. An example of a piercing and sampling probe is disclosed in U.S. Pat. No. 5,354,537 to Moreno. The subject matter of the patent to Moreno is incorporated herein by reference. Another example of a sampling probe is disclosed in U.S. Pat. No. 5,178,019, issued Jan. 12, 1993, to Keiter. The subject matter of the patent to Keiter is incorporated by reference herein. An example of a liquid level sensing probe and control circuit is disclosed in U.S. Pat. No. 5,493,922, to Ramey et al. The subject matter of the patents to Hulette et al., to Moreno, to Ramey et al. and to Keiter are each assigned to the same assignee as the present application.
Automated sample handling systems are known which automatically dispense fluid samples, such as blood plasma and reagents, into a reaction well of a cuvette. Such instruments are useful in the field of biochemical analysis for measuring blood clotting times and for automatically carrying out chemical assays. An automated sample handling system for carrying out blood and plasma bioassays is described in U.S. Pat. No. 5,236,666, to Hulette et al.
In this particular system, fluid samples, such as blood or plasma, are stored in containers, such as test tubes, which are vacuum sealed by way of a rubber septum that must be pierced in order to withdraw a measured amount of the sample for testing purposes. U.S. Pat. No. 5,354,537 to Moreno, discloses an example of a piercing and sampling probe suitable for piercing and sampling a measured amount of liquid.
The Hulette et al. system also includes a temperature controlled housing provided for storing fluid samples and reagents at a relatively cool temperature for preventing degradation of the samples and reagents prior to sample analysis. The temperature controlled housing typically maintains the fluid samples and reagents at a temperature of 10.degree. C. The actual analyses are generally carried out at 37.degree. C. (98.6.degree. F.), standard human body temperature. Accordingly, it is necessary to heat the fluid sample and reagents to 37.degree. C. prior to analysis. U.S. Pat. No. 5,178,019, to Keiter, discloses a sample probe device useful for heating fluid samples and reagents prior to analysis.
The piercing and sampling probes in the Hulette et al. system are raised and lowered in operation by a robotic arm which maneuvers a probe between reagent containers and a reaction cuvette for automatically aspirating and dispensing reagents. The surface of a liquid, whether a sample or reagent, is detected for accurately controlling movement of the probe. Basically, the surface of the liquid is detected by detecting a change in capacitance of the probe with respect to the chassis of the automated blood/plasma sampling system.